1. Field of the Invention
The present invention relates to a voltage detection unit including a circuit board formed with circuits for low voltage and circuits for high voltage.
2. Description of the Related Art
Some recent vehicles mount a cell pack in which secondary cells are connected in series. A voltage detection unit that detects an output voltage of each secondary cell is used in order to measure electric power output by the cell pack.
FIG. 7 is a circuit diagram illustrating a state in which voltage detection units are used for secondary cells.
As illustrated in FIG. 7, a cell pack includes secondary cell blocks CB1, CB2, . . . , and CBn connected in series. Voltage detection units 511, 512, . . . , and 51n are connected to the secondary cell blocks CB1, CB2, . . . , and CBn, respectively, and the voltage detection units 511, 512, . . . , and 51n detect output voltages of the corresponding secondary cell blocks CB1, CB2, . . . , and CBn, respectively.
FIG. 8 is an equivalent circuit of a circuit board of a conventional voltage detection unit.
In the voltage detection unit 51n, schematically two circuits are provided on the circuit board. One circuit is a circuit 500 for high voltage, and the other circuit is a circuit 550 for low voltage. Japanese Patent Application Laid-open No. 2011-229397 discloses a circuit board provided with a circuit for high voltage and a circuit for low voltage.
The circuit 500 for high voltage illustrated in FIG. 8 is connected to the secondary cell block CBn as a high-voltage power supply, and a voltage detection circuit detects voltages of a cell 1, a cell 2, . . . , and a cell m constituting the secondary cell block CBn. The voltage detection circuit outputs the detected voltages to a logic circuit to be driven by receiving voltage (electric power) from a 5 V power supply circuit. The 5 V power supply circuit, to which the voltage of the secondary cell block CBn is applied, supplies a rated voltage of 5 V to the logic circuit and a communication IC. The logic circuit converts the voltages of the cell 1 to the cell m input from the voltage detection circuit into binary signals and outputs the binary signals to the communication IC 600.
A 5 V power supply circuit of the circuit 550 for low voltage illustrated in FIG. 8, to which electric power is supplied from a vehicle's battery, supplies electric power with a rated voltage of 5 V to the logic circuit and the communication IC. A CPU receives signals from the communication IC 600, performs various kinds of operations, and outputs resultant output signals to an I/F.
The voltage detection unit 51n illustrated in FIG. 8 requires to take measures against the following matter. Specifically, the communication IC 600 that relays transmission and reception of signals between the circuit 500 for high voltage and the circuit 550 for low voltage is required to operate at a high clock frequency, and measures are required to be taken against harmonic noise that occurs at the communication IC 600 propagating therefrom to the circuit 500 for high voltage and the circuit 550 for low voltage.
As an example of the measures, a ground (hereinafter, referred to as a GND) in the pattern of the circuit 550 for low voltage is formed as a solid pattern (Japanese Patent Application Laid-open No. 2002-368355 discloses that the GND is formed as the solid pattern, for example).
FIG. 9 is a circuit diagram of the circuit board of a conventional voltage detection unit.
In the pattern of the circuit 550 for low voltage in FIG. 9, a GND 553 is formed as a solid pattern on the surface of a substrate except parts formed with a signal line 551 and an insulating part 552. By thus forming the GND 553 of the circuit 550 for low voltage as the solid pattern and capacitively coupling the signal line 551 and the GND 553, noise-resistance performance of the circuit 550 for low voltage against harmonic noise radiated from the communication IC 600 can be increased.
The circuit board of the conventional voltage detection unit in FIG. 9 takes no measures for the circuit 500 for high voltage. In other words, in FIG. 9, in the circuit 500 for high voltage, GNDs 503 are formed on the circuit board as thin patterns (lines), and insulation between the GNDs 503 and signal lines 501 is achieved by an insulating part 502. Although it can be considered that the GNDs 503 in the pattern of the circuit 500 for high voltage are formed as a solid pattern similarly to the circuit 550 for low voltage, it is generally considered that this measure is not effective. This is because, as illustrated in FIG. 8, the circuit 550 for low voltage causes the GND 553 to be conductively connected to a vehicle's body ground to stabilize its GND level to be zero, whereas the circuit 500 for high voltage causes the GNDs 503 to be connected to lowest potential of the secondary cell block CBn to make its GND level the lowest potential (a finite value). This fact means that the GND level of the circuit 500 for high voltage fluctuates depending on the state of the secondary cell block CBn. The circuit 500 for high voltage, the GND level of which is thus not stable, is questionable in noise-resistant performance, even if the GNDs 503 of the circuit 500 for high voltage are formed as the solid pattern. For this reason, the circuit 500 for high voltage with the GNDs 503 formed as the solid pattern has not been employed.